Various types of bore screens have been developed to date; including slotted casing, wire mesh, perforated sheet metal and wire wrapped screens. These screens exhibit individual merits and demerits, but one feature which is common to all is that a given screen cannot be adjusted to accommodate different particle size ranges. This means that differently sized screens must be constructed and carried in stock to suit various conditions.
Of all the prior art bore screens, the wire wrapped screen is considered generally to offer the best features. This screen comprises a cylindrical body which is constructed as a metal cage and around which a wedge-profile wire is wrapped. A continuous length of the wire is wound onto the cage and successive convolutions of the wire are welded to the cage. The space between each pair of convolutions is constant on any one screen but it may be varied from screen-to-screen to accommodate different particle sizes.
Whilst the wire wrapped screen is deemed to be the most suitable for most situations, it is more expensive than other types of screens and, as previously mentioned, it cannot be adjusted on site to provide for screening of different particle sizes.
A bore screen which was developed to overcome this problem is disclosed in Australian patent Specification No. 234285, dated Dec. 15, 1959, and, as described, it comprises a slotted metal cylinder around which a plurality of plastics material annular rings are stacked and clamped. Each ring is formed on one face with a plurality of cylindrical projections, and on its other face with three times as many cylindrical recesses. The recesses are located on the same pitch circle diameter as the projections, but the angle between adjacent projections is three times greater than that which exists between adjacent recesses. When one ring is stacked upon another, the projections of one ring locate within every third recess in the other. Thus, the rings are stacked to form a cylindrical structure and the rings, which extend about a common axis, are spaced from one another by the projections on each ring bottoming in recesses in each adjacent ring.
A special feature of this screen is that, while all of the projections on each of the rings have the same length, each recess has one of three possible depths and every third recess has the same depth. As a consequence, when one ring is turned about the common axis and relative to an adjacent ring, the space between the two rings is varied, and this feature permits a given screen construction to be used in different situations where various particle sizes may be encountered.
However, a problem which exists in relation to the above described screen is that the projections bottom only in every third recess and with such broadly spaced support between the rings, the rings tend to distort and assume a wavy form when clamped together. This in turn results in non-uniform and unpredictable spacing between the rings.